• KSCE Journal of Civil and Environmental Engineering Research
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• v.14 no.6
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• pp.1357-1363
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• 1994

A Monte-Carlo simulation model is developed to predict size distribution produced by the coalescence of air bubbles in turbulent shear f1ow. The simulation consists of generating a population of air bubbles into the initial positions at each time step and tracking them by simulating motions and checking collisions. The radial displacement of air bubbles in the simulation model is produced by numerically solving an advective diffusion equation. Longitudinal displacements are generated from the logarithmic flow velovity distribution and the bubble rise velocity. Collision of air bubbles for each time step is detected by a geometric test using their relative positions at the beginning of the time step and relative displacements during the time step. At the end of the time step, the total number of bubbles, their positions, and sizes are updated. The computer program is coded such that minimum storages for sizes and positions of bubbles are required.

• Journal of the Korean Institute of Telematics and Electronics
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• v.27 no.3
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• pp.101-107
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• 1990

In this work, one-dimensional simulation is carried out for PT-GaAs Schottky barrier diodes with finite difference method. Shockley's semiconductor governing equations: Poisson equation and current continuity equation are discertized, and linearized by Newton-Raphson method. The linear system of equation is solved by Gaussian elimination method until convergence is achieved. The boundary condition for this equation is taken from thermionic emission-diffusion theory. Simulation is done for PT-GaAs epitaxial-layer Schottky barrier diodes. The claculated results of electron and potential distribution are shown. Simulation results show exellent agreement with experiments.

• Applied Chemistry for Engineering
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• v.10 no.4
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• pp.509-514
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• 1999

In Preparing $BaTiO_3$ powder under hydrothermal conditions, effects of reaction period feedstock concentration and mixing rate on crystallinity, mean size and size distribution of particles were studied. Experimental results showed that the particle size became smaller with its narrow distribution as the reaction period and mixing rate increased, but feedstock concentration decreased in contrast with the results based on the classical nucleation-growth model. From these results, $BaTiO_3$ particles seem to be prepared hydrothermally through a multiple reaction procedure that includes dissolution, precipitation, hydrolysis-condensation, aggregation, diffusion and transformation.

• Applied Chemistry for Engineering
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• v.3 no.1
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• pp.119-129
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• 1992

The rates of mass transfer with the alkaline hydrolysis of ethyl acetate and n-butyl acetate were measured by using a modified Lewis cell. The rates of mass transfer with chemical reaction were independent of the speed of agitation, and the reaction enhancement factors were independent of the ionic strength. The second order reaction rate constants of ethyl acetate and n-butyl acetate could be obtained from an approximate solution of a diffusion equation by film theory, and their values were $0.041m^3/kg\;mol{\cdot}s$ and $0.338m^3/kgmol{\cdot}s$, respectively.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.11 no.3
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• pp.429-436
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• 2000

In this paper, the performance of a orthogonal noncoherent FH-BFSK communication system in the presence of partial band noise jamming is analyzed. Also bit error rate(BER) is studied when jamming bandwidth ratio to overall spread spectrum bandwidth,$\rho$, changes according to processing gain(PG). The performance is investigated by numerical analysis and computer simulation of SPW. Even if PG is high, required performance could not gain because error floor occurs when JSR is 10 dB and SNR is under 10 dB. PG can be obtained to acquire a required BER according to $\rho$ when SNR is above 12 dB.

• The Journal of the Korea institute of electronic communication sciences
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• v.7 no.2
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• pp.333-339
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• 2012

Global flow of communication is a trend of high speed, digitalization, and high-capacity. Furthermore, spread spectrum method has been dominantly utilized to efficiently use the frequency which is the scarce resource. The PLL (Phase Lock Loop) which is a widely used frequency synthesizer in communication systems has few problems such as status interferences and hence, this study utilized the DDS (Direct Digital Synthesis) which is a digital device that can minimize the problems of PLL for the study on the performance evaluation of high speed frequency hopping system design. We designed a system that practices high speed frequency hopping and interprets improvement of error-rates and evaluated its performance.

• Journal of the Korean Institute of Gas
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• v.23 no.6
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• pp.74-80
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• 2019

Hydrogen refueling stations that use compressed hydrogen at high pressure provide safety distances between facilities in order to ensure safety. Most accidents occurring in hydrogen stations are accidental leaks. When a leak occurs, various types of ignition sources generate a jet flame. Therefore, the analysis of leaked gas diffusion and jet flame due to high pressure hydrogen leakage is one of the most important factor for setting the safety distance. In this study, the leakage accidents that occur in the hydrogen refueling station operated in high pressure environment are simulated for various leakage source sizes. The results of this study will be used as a reference for the future safety standards.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.479-482
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• 2008

In order to more closely examine the vacuum arc phenomena, it is necessary to predict the magnetohydrodynamic (MHD) characteristics by the multidisciplinary numerical modeling, which is coupled with the electromagnetic and hydrodynamic fields, simultaneously. In this study, the thermal-fluid characteristics of high current vacuum arcs were calculated by a commercial multiphysics package, ANSYS, in order to obtain Joule heat, Lorentz force and the interactions with flow variables. We assumed the diffused-mode arc within an AMF vacuum interrupter. It was found with four different currents that the temperature distributions on the anode surface are diffused uniformly without concentration in 7kA for both types (cup and coil-type). But the arc plasma transition and an increase of thermal flux density for increasing the applied current have caused the change of temperature distributions on the anode surface. We should need further studies on the two-way coupling method and radiation model for arc plasmas in order to accomplish the advanced analysis method for multiphysics.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.7
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• pp.1047-1054
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• 2002

To improve the fuel consumption and exhaust emission for gasoline engines, GDI(Gasoline Direct Injection) system was spotlighted to solve above requirements. Thus, many researchers have been studied to investigate the spray characteristics and the mixture formation of GDI injector. In this study, we tried to study the spray characteristics of a gasoline direct injector by using entropy analysis and PlV methods. The entropy analysis is based on the concept of statistical entropy, and it identifies the degree of homogeneity in the fuel concentration. The PlV method was adopted to determine the fluid dynamics information at the spray. From the applied results on a direct injection gasoline spray, we could find that the direct diffusion phenomena was a dominant factor in the formation of a homogeneous mixture at downstream of GDI spray especially under vaporizing ambient conditions, and mixing phenomena was also progressed by momentum exchange with induced air. In addition, the correlation between entropy and vorticity strength enabled to find their relation.

• Journal of Korean Society for Atmospheric Environment
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• v.13 no.5
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• pp.383-396
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• 1997

Numerical prediction of the pollutant dispersion over a two-dimensional hilly terrain is presented. The dispersion model used in the present work is based on the gradient diffusion theory and the finite-volume method on a non-orthogonal boundary-fitted grid system. The numerical model is validated by comparing the results with the available experimental data for the flat-floor dispersion within a turbulent boundary-layer. The numerical error analysis is performed based on the guideline of Kasibhatla et al.(1988) for the elevated-source dispersion in the flat-floor boundary layer having a power-law velocity and linear eddy-diffusivity profile. The influences of the two-dimensional hilly terrain on the dispersion from a continuously released source are numerically investigated by changing the emission locations and heights. It is found that the distributions of ground-level concentration are strongly influenced by the source location and the emission height. Hence, the terrain amplification factor is greatly enhanced when the pollutant source is located within a flow separation region. Dispersion from a source of short duration is also simulated and the duration time of the pollutant is compared at several downstream locations on a hilly terrain. The results of the numerical prediction are applied to the evaluation of environmental impacts due to the automobile exhausts at the seashore highway with a parallel mountain range.